Recently, there are growing demands for energy harvesting apparatus that generate power corresponding to the external environment in order to allow users to use portable devices such as smartphones, laptop personal computers (PCs), tablet PCs and other external devices even while they are on the go and do not have access to commercial power sources. Such energy harvesting apparatus include those having solar cells that generate power using solar energy, and those that generate power using geothermal energy.
The output power and output voltage of the power generated by such energy harvesting apparatus varies depending on the external environment. When an external device is connected to the energy harvesting apparatus via an interface conforming to a certain standard such as universal serial bus (USB), however, there is a problem of power delivery failure when the output voltage becomes less than a certain level.
One of known energy harvesting apparatus that counter such a problem has a feedback circuit connected to solar cells, wherein the feedback circuit monitors the voltage of the solar cells so that the solar cells are controlled to provide constant output voltage (see, e.g., PTL 1).